Displays are used on notebook PCs, televisions, and other electronic devices. Like most electronic devices, displays must be calibrated to accurately display video and graphic images. For example, the gamma response of a display is calibrated for optimum viewing and operation. Without proper calibration, the image on the display can be different from the original image captured by a camera.
A “gamma transfer” function characterizes the relationship between the light captured by a camera and the corresponding pixel voltages. FIG. 1A is a graph 20 of one such relationship, V=LIN1/γ for γ≈2. Such a gamma transfer function is exemplary of the light captured by a vacuum-tube camera. When rendering an image from these voltages, the display equipment must account for this non-linear relationship. Television cathode ray tubes (CRTs) have a gamma transfer function almost the inverse of the gamma transfer function of the vacuum-tube camera. When a CRT displays a video image captured from a vacuum-tube camera, the gamma transfer function of the CRT display essentially cancels the response of the vacuum-tube camera gamma transfer function. FIG. 1B is a graph 30 of one such relationship for a CRT display, where LOUT=Vγ for γ≈2. The CRT's “gamma correction” is thus inherent, requiring little, if any, additional processing to accurately display video images. In other words, a video image captured by a camera with the response graph 20 and then displayed by the CRT with the response graph 30 will have a substantially linear output.
Other types of displays, such as liquid crystal displays (LCDs), however, have transfer characteristics different from CRTs and therefore need different gamma correction to accurately display video images. FIG. 2 is a graph 40 of one such relationship. Because displays differ, by manufacturer, by part number, and even by individual panel, each requires it own gamma correction so that the display achieves a linear response, or any other desired response.
Gamma correction is provided as a gamma correction signal supplied to source driver chips that drive the display. The gamma correction signal is supplied using an appropriate gamma application circuit. During the assembly of a conventional display panel, the gamma correction signal can be calibrated by electronically adjusting parameters associated with the gamma application circuit. In some processes, a technician views a test image on the display and manually adjusts the gamma application circuit through empirical trial and error until the image is properly displayed. In other processes, the gamma correction signal calibration can be performed by monitoring the display panel and inputting detected display characteristics into a software algorithm to determine the proper gamma correction signal. The determined proper gamma correction signal is then set by the gamma application circuit. The proper gamma correction signal is then stored in non-volatile storage or set by a resistor string on the display controller board for the life of the display.
Conventional gamma application circuits use a Class AB amplifier to generate the proper gamma correction signal that is provided to source driver chips. FIG. 3 illustrates an exemplary conventional gamma application circuit 10. A digital-to-analog converter (DAC) 2 receives as input a digital code representative of the proper gamma correction signal stored in memory. The DAC 2 outputs a converted analog signal to a first input of an amplifier 4. The amplifier 4 is a Class AB operational amplifier. A second input of the amplifier 4 is a feedback signal. The amplifier 4 is supplied with an analog power supply voltage AVDD. An output of the amplifier 4 is the gamma correction signal that is supplied to each of one or more source driver chips 6. The source driver input impedance can be modeled as an equivalent resistance and as a capacitance to AC ground.
The gamma correction signal is substantially constant. The gamma application circuit 10 includes a local feedback from the output of the Class AB amplifier 4 to the second input of the Class AB amplifier 4.
The gamma correction signal distorts the gamma transfer function of the source driver chip to correct for the non-linear behavior of the display. Distorting the gamma transfer function of the source driver chip adjusts the response of the display. In some applications, the display response is adjusted to achieve a linear transfer function.
The output stage of a typical Class AB amplifier includes two complimentary transistors configured for sourcing and sinking current. The transistors in a Class AB amplifier operate in the linear mode. The power efficiency of the output stage of the typical Class AB amplifier is at best 50%.